The invention relates to monitoring physiological conditions as an indicator of shock. More specifically, the invention relates to monitoring of blood flow in tissues as an indicator of shock.
Shock is a clinical syndrome in which blood flow to the capillary beds (the perfusion) is decreased. Shock occurs in about 1 million patients/year in the United States and a total of about 3 million patients/year are at risk. Shock occurs when arterial pressure and subsequently tissue blood flow drop so low that the amount of delivered oxygen is inadequate to meet the metabolic needs of the tissue.
During shock, the body directs blood to the heart and the brain, often at the expense of xe2x80x9csacrificialxe2x80x9d organs such as the liver, skin, muscle, and gut. Prolonged shock may diminish blood flow to the gut such that the normal intestinal barrier function is disrupted and gut-derived bacteria and endotoxins are translocated to other organs via the blood. This, in turn, may lead to bacteremia, sepsis, inflammatory response and ultimately multi-organ failurexe2x80x94one of the major causes of patient mortality.
Conventional therapy for shock involves resuscitation. Resuscitation therapy is directed toward first assuring that oxygen is being supplied to the patient and that it is being transported through the circulation to the organs to support life. Circulatory distress is addressed with the infusion of fluids and pharmacological agents (inotropes) to increase cardiac output. Therapy is typically titrated to attain a target heart rate (HR), systolic blood pressure (BP), mean arterial blood pressure (MAP), urine output, and normal arterial pH. Cardiac output (CO) may also be monitored. While these conventional parameters are thought to give an indirect indication of tissue oxygenation, they correlate poorly with survival in critically ill patients (Astiz and Rackow, 1993; Shoemaker et al., 1993).
While the global, systemic parameters (HR, BP, CO, etc.) are readily accessible, these non-specific variables cannot tell if oxygen deprivation is occurring in one or more tissue beds or organs. Given the limitations of global monitoring, a number of local tissue monitoring techniques have been proposed to detect the onset of shock and provide an optimal xe2x80x9cend pointxe2x80x9d to guide therapy for complete resuscitation. Techniques have been proposed to monitor parameters (pO2, pH, pCO2, lactate levels, etc.) in sacrificial tissues that are susceptible to hypoperfusion, hypoxia and ischemia to provide an optimal xe2x80x9cend pointxe2x80x9d to guide resuscitation therapy. While these parameters are an attempt to assess the local tissue blood flow, and hence the oxygen delivery, these parameters also depend on metabolism and their respective arterial blood concentrations. Since during shock the blood supply is directed to the heart and the brain, often at the expense of the liver, skin, muscle and gut, these xe2x80x9csacrificialxe2x80x9d organs are thought to provide sites to monitor shock onset and resuscitation end points. The sacrificial organs are the first to develop hypoperfusion at shock onset and are the last to be restored after resuscitation. These prior methods, however, have not revealed an effective correlation between patient survival and outcome and are not well suited for rapid and simple use in a clinical setting. Therefore, a reliable monitor for gut ischemia is needed, because such measurements could significantly impact the management of shock patients.
The following patents are cited as background information herein, and to the extent necessary for a full and complete understanding of this invention, these patents are hereby incorporated herein by reference: U.S. Pat. Nos. 4,059,982, 4,852,027, 6,2221,025, 6,010,455, 5,792,070, 5,771,261, 5,769,784, 5,404,881, 5,335,669, 5,205,293, 4,859,078, 4,413,633, 4,392,005, 4,306,569, 3,818,895, 3,623,473 and Design Pat. No. 384,412.
An object of the present invention is to provide a shock monitoring apparatus. It is a particular object of certain aspects to use the shock monitoring apparatus to monitor for shock through measurement of rectal wall blood flow as a proxy for gut ischemia.
In accordance with a first aspect, a shock monitoring apparatus comprises a probe and a controller. Optionally, the apparatus comprises one or more additional probes or sensors. The probe typically functions to provide an input stimulus to an area of interest, such as to tissue in the rectum. That is, the probe transmits an input signal, e.g., heat, into the tissue region contacted by the probe. The input signal functions to perturb the tissue. The tissue functionally responds to such perturbations, and this functional response can be correlated with the physiological state of the tissue, e.g., low blood flow to the tissue, etc., as an indicator of the state-of-shock (SOS) in the patient. In certain embodiments, a reference probe is used to account for baseline fluctuations in the tissue temperature. The system measures the functional response of the tissue and transmits an output signal to a controller. The controller then typically performs one or more operations on the signal, e.g., recording, adding, subtracting, comparing, etc. In certain embodiments described here, the output signal is compared with tabulated values contained in the controller to calculate a blood flow value based on known blood flow values.
In accordance with preferred embodiments, a system for monitoring shock comprises an apparatus for supplying heat to tissue and measuring the thermal response in the tissue, which is functionally related to physiological conditions in the tissue, e.g., blood flow in the tissue, and an device for calculating a blood flow value. Optionally, the system comprises one or more additional probes or other sensors. Such apparatus for supplying heat to tissue are well known to those skilled in the art and include, but are not limited to thermistors, thermocouples, electric wires, etc.
In accordance with additional aspects, the heating apparatus may be electrically energized, or magnetically energized as the case may be, to elevate the temperature of the apparatus and/or the probe. In preferred embodiments, the heating apparatus is designed such that only the portion of the probe in contact with the tissue is heated.
The blood flow values may be representative of several indicators of shock including but not limited to blood flow in tissue, oxygen levels in the tissue, in pH, etc. In certain embodiments, the blood flow values are converted to State-Of-Shock (SOS) values to facilitate rapid clinical assessment of a patient""s condition. For example, if blood flow value is between 95-100% of non-shock blood flow value, e.g. the blood flow value in the absence of shock, an SOS value of xe2x80x9c1xe2x80x9d may be assigned. If the blood flow is between 85-95% an SOS value of xe2x80x9c2xe2x80x9d may be assigned and so on. It is preferred, but not required, that the SOS values are on a scale of xe2x80x9c1-5xe2x80x9d, where an SOS value of xe2x80x9c1xe2x80x9d represents little or no shock and an SOS value of xe2x80x9c5xe2x80x9d represents severe shock. One skilled in the art will recognize that the scaling of blood flow values is not limited to the xe2x80x9c1-5xe2x80x9d scale or that the percentages of the blood flow values necessarily are limited to the scaling described here.
In accordance with a method aspect, the shock monitoring apparatus is used to input a stimulus into the tissue, measure the response of the tissue to the stimulus, transmit and record the response of the tissue in an output signal, and output or display the results of the measurement for evaluation of the patient""s physiological state. The stimulus may comprise heat, an electric current, a voltage, or any other signal capable of perturbing a physiological condition indicative of blood flow, e.g., the temperature, of the tissue. The response of the tissue is typically measured using the probe itself. In other embodiments, the response of the tissue is measured using any of the sensors well known to those skilled in the art, such as those manufactured by Thermal Technologies Inc (Cambridge, Mass.) and Diametrics Medical, Inc. (St. Paul, Minn.).
The output signal typically represents a value functionally related to the response of the tissue to the input signal. For example, the output signal may reflect an amount of heat required to elevate the temperature of the tissue by a certain quantity, the amount of current required to elevate the temperature of the tissue by a certain quantity, the amount of power required to elevate the temperature of the tissue by a certain quantity, the amount of heat transferred from the probe to the tissue or from the tissue to the probe, the intrinsic thermal conductivity of the tissue, perfusion values, the amount of heat required to maintain a constant temperature, etc.
In accordance with preferred embodiments, the temperature of a heating apparatus, in contact with tissue, is elevated above the baseline temperature of the tissue. Such heating typically is performed by introduction of an electric current, e.g., an electrical signal, into an electric heater in contact with the tissue. An electrical signal is produced that is indicative of the amount of energy required to raise the temperature of the heating apparatus and the rate at which the heat from the apparatus is transferred to the tissue. Based on the values obtained, a blood flow value can be calculated. Without wishing to be bound by any scientific theory, a value indicative of shock may be the difference between a blood flow signal indicative of no shock and the signal from the current state of the tissue, e.g., a difference of zero would be representative of no shock. Therefore, relative changes in the blood flow value can be monitored as an indicator of functional changes in the tissue. After measurement of the output signal, the temperature of the heating apparatus is then lowered back to the baseline temperature of the tissue. The steps of elevating the temperature, recording the signal, and reducing the temperature to baseline are repeated continuously (or cyclically with an optional delay between cycles) to provide for online monitoring of a patient""s blood flow values. Reductions in the blood flow values from a base condition, e.g., blood flow values in the absence of shock, are indicative of the likelihood of the occurrence of shock. Therefore, changes in a patient""s blood flow values, during continuous monitoring of the patient, can allow physicians to undertake measures to prevent the onset of shock or to reduce the pathological and physiological damage that would occur in the absence of any intervention.
In accordance with preferred embodiments, the shock monitoring apparatus may be used to iteratively calculate blood flow values. Such systems typically comprise a probe in contact with tissue, e.g., a thermistor, a controller for introducing an input signal into the probe to perturb the tissue, e.g., a controller to cause the temperature of the thermistor to cyclically rise and fall, the rate of temperature rise in an initial time period within each energizing and deenergizing cycle is substantially a function of the intrinsic thermal conductivity of tissue in thermal contact with the thermistor. The controller also may transmit an output signal that can be used to iteratively calculate values for determining the blood flow value of the tissue. Such calculations may be performed using the controller itself or using an external calculating device such as a computer. Numerous calculations and operations may be performed on the output signal. In accordance with preferred embodiments, the output signal is used to calculate an intrinsic thermal conductivity. Without wishing to be bound by any scientific theory, the intrinsic thermal conductivity typically is represented by the temperature rise in an initial time interval. This intrinsic thermal conductivity is a function of the power provided to the probe to raise its temperature to a predetermined value, since more power typically introduces more heat. The intrinsic conductivity value is used to calculate a blood flow (perfusion) value indicative of shock.
In accordance with preferred embodiments, the calculated blood flow value (perfusion value) can be used to recalculate the calculated value of thermal conductivity. The recalculated conductivity value is used to recalculate the calculated value of the blood flow (perfusion). Such steps of calculating thermal conductivity, calculating blood flow values, recalculating thermal conductivity and recalculating blood flow values are typically repeated until the value for blood flow does not change substantially. That is, the iterative calculation can be performed until the perfusion values do not change by more than about 5%, preferably no more than about 1%, and most preferably no more than about 0.1%. For example, the calculation stops when successive thermal conductivity values and blood flow values differ by less than about 0.05%. Such values are referred to here as substantially converged blood flow values. After calculating the substantially converged blood flow values, an SOS value may be calculated and used as an indicator of shock. The calculated blood flow values (or SOS values) may be displayed or recorded for monitoring of a patient""s susceptibility to shock. The changes and variations in such values can be correlated with the likelihood of shock. Automated monitoring systems may be designed that alert clinical personnel when a patient""s SOS values are outside an acceptable range of SOS values. Thus, systems comprising the shock-monitoring device described here provide for continuous and automated monitoring of patient""s in a clinical setting.
The shock monitoring apparatus (and systems comprising the shock monitoring apparatus) disclosed here provides medical facilities the ability to monitor patients for the probability of shock onset. Such devices can aid in reduction of the mortality rate from shock and can also be used as an additional monitoring technique to assess the clinical status of patients.
Certain especially preferred aspects of the present invention may be summarized as follows:
One aspect of the present invention is directed to a system for monitoring shock comprising:
means for supplying heat to tissue in the inner wall of the rectum;
means for sensing in the tissue a thermal response functionally related to the perfusion of blood in the tissue; and
means for calculating a value indicative of shock as a function said thermal response. Preferably, the means for supplying heat to tissue comprises a thermistor. Advantageously, the sensor comprises a thermal diffusion probe. Alternatively, the sensor comprises an intraluminal probe.
Another preferred aspect of the present invention is directed to a shock monitor comprising:
a thermistor for thermal contact with tissue at a site on the inner wall of the rectum;
means for electrically energizing said thermistor to elevate the temperature of said thermistor above the baseline temperature of tissue at said site;
means for producing an electrical signal having a value functionally related to the electrical energy supplied to said thermistor and the rate at which heat from said thermistor is transferred in said tissue;
means for producing a signal indicative of shock as a function of said electrical signal.
Another preferred aspect of the present invention is directed to a shock monitor comprising:
thermistor means for thermally contacting living tissue at a site on the inner wall of the rectum;
means for electrically energizing and deenergizing said thermistor means cyclically to cause the temperature of said tissue to rise and fall cyclically;
means for producing a signal functionally related to the power used to energize said thermistor during each energizing and deenergizing cycle;
means responsive to the power related signal from said producing means for producing a signal during each energizing and deenergizing cycle as a function of perfusion in said tissue; and
means for computing a value for blood flow in said tissue indicative of shock during each energizing and deenergizing cycle as a function of the perfusion related signal. Preferably, the means for computing a value comprises a microprocessor. Advantageously, the means for computing a value comprises an embedded microdevice.
Another preferred aspect of the present invention is directed to a system for producing a signal indicative of shock comprising:
a thermistor for contacting the inner wall of the rectum to establish thermal contact with tissue at a site in the inner wall of the rectum;
control means for electrically energizing and deenergizing said thermistor cyclically to cause the temperature of said thermistor to cyclically rise and fall, the rate of temperature rise in an initial time period within each energizing and deenergizing cycle being substantially a function of the intrinsic thermal conductivity of tissue in thermal contact with said thermistor;
means for producing a signal functionally related to the power used to energize said thermistor during each energizing and deenergizing cycle; and
iterative calculating means for:
calculating intrinsic thermal conductivity in the initial time interval during each energizing and deenergizing cycle as a function of the temperature rise in the initial time interval and the power related signal produced by said producing means;
calculating perfusion in a subsequent time interval during each energizing and deenergizing cycle as a function of the calculated value of intrinsic thermal conductivity;
recalculating intrinsic thermal conductivity in the first time interval using the calculated value of perfusion;
recalculating perfusion in the subsequent time interval using the recalculated value of intrinsic thermal conductivity; and
recalculating values for intrinsic thermal conductivity and perfusion, in alternating fashion, until the recalculated values of perfusion converge to a substantially unchanging value, using in each recalculation of perfusion the previously recalculated value of intrinsic thermal conductivity and in each recalculation of intrinsic thermal conductivity the previously recalculated value of perfusion.
Another preferred aspect of the present invention is directed to a method of monitoring shock in a living subject comprising the steps of:
supplying heat to tissue in the inner wall of the rectum;
sensing in the tissue a thermal response functionally related to the perfusion of blood in the tissue; and
calculating a blood flow value indicative of shock as a function said thermal response. Preferably, the heat is supplied using a thermistor. Advantageously, the blood flow value is calculated by comparing the thermal response with a table of thermal response values.
Another preferred aspect of the present invention is directed to a method of monitoring shock comprising the steps of:
contacting the inner wall of the rectum with electrically energizable thermistor means to establish a heat transfer path between said thermistor means and tissue at a site along the inner wall of the rectum;
energizing said thermistor means to elevate the temperature of said thermistor means above the baseline temperature of said tissue;
sensing the thermal response in said tissue to the application of heat from said thermistor means; and
calculating a blood flow value indicative of shock as a function of the thermal response in said tissue sensed in said sensing step. Preferably, the blood flow value is calculated by comparing the thermal response with a table of thermal response values. Advantageously, said calculating step comprises the steps of:
calculating intrinsic thermal conductivity in a first time interval during said energizing step;
calculating perfusion in a subsequent time interval during said energizing step using the calculated value of intrinsic thermal conductivity;
recalculating values for intrinsic thermal conductivity and perfusion in alternating fashion, until the recalculated values of perfusion converge to a substantially unchanging value, using in each recalculation of perfusion the previously calculated value of intrinsic thermal conductivity and in each recalculation of intrinsic thermal conductivity the previously calculated value of perfusion; and
calculating a blood flow value indicative of shock as a function of the converged value of perfusion.
Another preferred aspect of the present invention is directed to a method of monitoring shock comprising the steps of:
contacting the inner wall of the rectum with a thermistor to establish a thermal transfer path with tissue at a site in the inner wall of the rectum;
electrically energizing and deenergizing said thermistor cyclically to cause the temperature of tissue in thermal contact with said thermistor to cyclically rise and fall, the rate of temperature rise in an initial time period within each energizing and deenergizing cycle being substantially a function of the intrinsic thermal conductivity of tissue in thermal contact with said thermistor;
producing a signal functionally related to the power used to energize said thermistor during each energizing and deenergizing cycle;
calculating intrinsic thermal conductivity of tissue at said site in an initial time interval during each energizing and deenergizing cycle as a function of the temperature rise and said power related signal in the energizing and deenergizing cycle;
calculating perfusion in a subsequent time interval during each energizing and deenergizing cycle as a function of the calculated value of intrinsic thermal conductivity;
recalculating intrinsic thermal conductivity in said first time interval using the calculated value of perfusion;
recalculating perfusion in said subsequent time interval using the recalculated value of intrinsic thermal conductivity;
recalculating values for intrinsic thermal conductivity and perfusion, in alternating fashion, until the recalculated values of perfusion converge to a substantially unchanging value, using in each recalculation of perfusion the previously recalculated value of intrinsic thermal conductivity and in each recalculation of intrinsic thermal conductivity the previously recalculated value of perfusion; and
processing said substantially unchanging perfusion value during each energizing and deenergizing cycle to provide a blood flow signal indicative of shock.
Another preferred aspect of the present invention is directed to a system for producing a signal indicative of shock comprising:
thermistor means for thermally contacting living tissue;
means for electrically energizing and deenergizing said thermistor means cyclically to cause the temperature of said tissue to rise and fall cyclically;
means for producing a signal functionally related to the power used to energize said thermistor during each energizing and deenergizing cycle; and means responsive to the power related signal from said producing means for producing a signal indicative of shock during each energizing and deenergizing cycle. Preferably, the system further comprises a blood flow model wherein said signal indicative of shock is a function of the relationship of said power related signal to said blood flow model. Advantageously the system further comprises a model that relates temperature and power to tissue blood flow wherein said signal indicative of shock is a function of the relationship of said power related signal and the change in temperature produced by said energizing and deenergizing means to a blood flow value determined by said model. In addition, the system will utilize the relationship of said power related signal and the change in temperature produced by said energizing and deenergizing means is the ratio of said power related signal to said change in temperature. In such systems the thermistor means may comprise means for thermally contacting a site on the inner wall of the rectum.